This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-094573, filed Mar. 30, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a charged particle beam system such as an electron beam lithography system used in pattern writing or pattern transfer to a substrate and, more particularly, to a chamber of this charged particle beam system.
As the integration degree of LSI devices increases, demands for the writing accuracy and throughput of a lithography system used in the fabrication of LSIs are becoming more and more strict. At present, a charged particle beam system such as an electron beam lithography system used as a lithography system employs lithography schemes such as a VSB (Variable Shaped Beam) scheme, CP (Character Projection) scheme, or pattern transfer scheme, in order to ensure sufficient throughput.
FIG. 1 is a schematic view of a VSB electron beam lithography system. This system will be described below in order. An electron beam emitted from an electron gun 51 enters into an electron optics system 50. More specifically, the electron beam passes through a condenser lens 52 and illuminates a first shaping aperture 53. This first shaping aperture 53 is, e.g., a rectangle 100 xcexcm square. Hence, the electron beam passing through this first shaping aperture 53 is shaped into a square of 100 xcexcm side.
The shaped electron beam is projected onto a second shaping aperture 56 through a projection lens 54. This second shaping aperture 56 is a square aperture of, e.g., 100 xcexcm side. A beam shaping deflector 55 is disposed upstream of the second shaping aperture 56. The propagating direction of the electron beam can be changed by applying an appropriate voltage to this beam shaping apparatus 55. Consequently, the position of the first shaping aperture image projected onto the second shaping aperture 56 can be changed.
By thus changing the projection position, the overlap of the first shaping aperture image and the second shaping aperture changes, so square beams differing in size can be shaped. The beam shaped into a square is reduced by a reduction lens (not shown) and positioned by a sub deflector 57 and a main deflector 59. Furthermore, the focusing position of the beam is determined by an objective lens, and the beam arrives at a predetermined position of a specimen 60.
The specimen 60 can move by moving mechanisms called an X stage and Y stage. By these moving mechanisms, writing is performed by a step-and-repeat scheme or a continuous stage moving scheme. The continuous stage moving scheme will be described below. In this continuous stage moving scheme, a pattern to be written is divided into stripes, and each stripe is written. During the writing of each stripe, a stage (e.g., the X stage) is continuously moved. When one stripe is completely written, a stage (this time the Y stage perpendicular to the X stage) is moved one step by the width of a stripe to write the next stripe. In this way, an LSI pattern having a large writing area can be written efficiently at a high speed.
In this continuous stage moving scheme, the position of a specimen changes every moment, so this movement of each stage must be added to information about a position at which the beam should arrive. To this end, the position of each stage at each time must be accurately known. A laser interferometer is commonly used to measure this stage position.
FIG. 2 is a schematic view for explaining the principle of a laser interferometer. A laser beam emitted from a laser oscillator 61 is split into two directions by a beam splitter 63 and incident on a movable mirror 64 and a fixed mirror 62 via a reflective mirror 67. The laser beam reflected by the fixed mirror 62 and further by the reflective mirror 67 and the laser beam reflected by the movable mirror 64 merge on the beam splitter 63 to generate interference fringes. This interference component is detected by a detector 65 and analyzed by an analyzer 66 to find the position of the movable mirror 64.
This movable mirror 64 is set on a specimen stage. The fixed mirror 62 is fixed to, e.g., the ceiling of a chamber.
The position of this fixed mirror must always be fixed in the entire laser interferometer system. Otherwise, changes in the position of the movable mirror cannot be accurately detected any longer.
On the other hand, the walls of the chamber thermally expand or contract owing to temperature changes. Therefore, if the temperature of the environment in which the chamber is set changes, the position of the fixed mirror fixed to an inner wall of the chamber also changes.
This chamber is usually made of stainless steel. A coefficient xcex1 of linear expansion of stainless steel is 15xc3x9710xe2x88x926/xc2x0 C. near a room temperature. This means that the linear expansion of 1-m long stainless steel is 15 xcexcm/xc2x0 C.
Assume that the temperature of the installation environment of the charged particle beam system is controlled within the range of xc2x10.1xc2x0 C. In this state, a positional change of about xc2x10.225 xcexcm occurs in a position apart by 150 mm from the center of the stainless steel chamber. Accordingly, the position of the fixed mirror fixed to this position also changes by 0.225 xcexcm. As described above, the landing position of the electron beam is determined by referring to the stage position information. Consequently, an error corresponding to the positional change of the fixed mirror is produced, and this significantly degrades the writing accuracy. This is a fatal drawback for writing required to have a nanometer-level dimensional accuracy.
It is an object of the present invention to reduce positional changes of a fixed mirror of a laser interferometer, which is fixed to an inner wall of a chamber, in accordance with temperature changes of a charged particle beam system.
To achieve the above object, a charged particle beam system according to the first aspect of the present invention comprises
a chamber having a space to accommodate a specimen therein and a first opening to communicate with the space in an upper surface,
a table placed, immediately below the first opening, on a bottom surface of the chamber, the table being movable in at least one direction,
a laser interferometer set in the chamber, the laser interferometer comprising
a laser oscillator placed along the one moving direction of the table,
a movable mirror placed on that side surface of the table, which opposes the laser oscillator,
a beam splitter placed on a line connecting the laser oscillator and the movable mirror, and
a fixed mirror perpendicular to the line connecting the laser oscillator and the movable mirror and fixed, immediately above the beam splitter, on the upper surface of the chamber,
an optical lens barrel having a second opening communicating with the first opening of the chamber, and coupled with the chamber so as to match the first and second openings, and
a beam gun set on an inner upper surface of the optical lens barrel to irradiate the specimen placed on the table set on the bottom surface of the chamber with a charged particle beam through the first and second openings,
wherein at least an upper wall portion from the opening to a fixed portion of the fixed mirror of the chamber is made of an invar alloy.
A charged particle beam system according to the second aspect of the present invention is characterized in that the chamber is made of an invar alloy in the arrangement of a charged particle beam system according to the first aspect.
A chamber of a charged particle beam system according to the third aspect of the present invention comprises
a housing having a space to accommodate a specimen therein and an opening to communicate with the space in an upper surface,
a table placed, immediately below the first opening, on a bottom surface of the housing, the table being movable in at least one direction, and
a laser interferometer set in the housing, the laser interferometer comprising
a laser oscillator placed along the one moving direction of the table,
a movable mirror placed on that side surface of the table, which opposes the laser oscillator,
a beam splitter placed on a line connecting the laser oscillator and the movable mirror, and
a fixed mirror perpendicular to the line connecting the laser oscillator and the movable mirror and fixed, immediately above the beam splitter, on the upper surface of the housing,
wherein at least an upper wall portion from the opening to a fixed portion of the fixed mirror of the housing is made of an invar alloy.
The above first to third aspects desirably have the arrangements such that the invar alloy is invar or super invar.
The laser interferometer further comprises a detector for receiving reflected light from the movable mirror and the fixed mirror via the beam splitter and detecting an interference component of the reflected light.
Also, in the first and second aspects, at least an under-half portion of the optical lens barrel are desirably made of an invar alloy.
In the present invention, a region of the chamber from the opening to a portion where the fixed mirror is set is made of a material, called an invar alloy, having a small coefficient of linear expansion. Therefore, it is possible to reduce positional changes of the fixed mirror of the interferometer caused by thermal expansion and contraction.
An alloy so-called invar is one material having a small thermal expansion coefficient near a room temperature. Invar is an iron alloy containing about 36 wt % of nickel. FIG. 4 is a graph showing a coefficient xcex1 of linear expansion which changes in accordance with an amount of nickel added to iron. This graph shows that the coefficient of linear expansion of an iron alloy containing about 36 wt % of nickel is low. The coefficient xcex1 of linear expansion near a room temperature of this 36 nickel-iron alloy is about 1.2xc3x9710xe2x88x926/xc2x0 C. (FIG. 3). Likewise, an iron alloy so-called super invar containing 32 wt % of nickel and 5 wt % of cobalt has a very small coefficient of linear expansion of about 0.4xc3x9710xe2x88x926/xc2x0 C. These invar and super invar are generally called invar alloys.
Also, the chamber is used as a vacuum vessel. Therefore, the material of the chamber must well resist the atmospheric pressure or the weights of parts mounted on the upper portion of the chamber. Additionally, the material must have satisfactorily small degassing to maintain high-quality vacuum. Furthermore, the material is required to be processable into the shape of a vessel and allow vacuum flanges and ports to be attached. An invar alloy has sufficient rigidity and hence has sufficient strength even when used as a vacuum vessel.
The present invention is characterized in that the whole chamber or a region of the chamber from the opening to a portion where the fixed mirror is set is made of invar or super invar described above, and the fixed mirror as a component of the interferometer is fixed to this material.
In the present invention, the chamber is manufactured using invar or super invar having thermal expansion much smaller than stainless steel or iron. Particularly, the fixed mirror of the parts of the interferometer is fixed to an inner wall of this chamber. Accordingly, positional changes of the fixed mirror caused by thermal expansion of the chamber can be made one or more orders of magnitude smaller than that of a conventional stainless steel or iron chamber.
Since the stage positional information can be accurately known in this way, high-accuracy lithography can be performed without being influenced by environmental temperature changes. For example, assume that a fixed mirror is set in a position 150 mm away from the center of a super invar chamber. If an environmental temperature change is xc2x10.1xc2x0 C., a positional change by thermal expansion is xc2x16 nm. This value is {fraction (1/40)} a positional change of xc2x10.25 xcexcm of the above-mentioned stainless steel chamber.
This positional change of about xc2x16 nm has no large influence on the accuracy of photolithography required to have a nanometer-level dimensional accuracy. Therefore, high-accuracy photolithography is implemented regardless of the environmental temperature.
Another effect of the use of invar or super invar is the magnetic shielding effect. No magnetic shielding effect can be obtained by the conventional stainless steel chamber in a static field, because the magnetic permeability of stainless steel is small. Hence, permalloy as a high-magnetic-permeability material is pasted on the inner or outer walls of the chamber. The thickness of the permalloy plate is usually about 1 mm. This can suppress deterioration of the photolithography accuracy occurring when the orbit of an electron beam is changed by an environmental magnetic change.
The magnetic permeability xcexc of invar or super invar is 2,000 to 3,000 (FIG. 4). This value is 10 times or more the magnetic permeability of iron. Therefore, a magnetic shielding effect much superior to that of the iron chamber can be realized. On the other hand, this magnetic permeability is one order of magnitude smaller than that of permalloy. However, the wall thickness of the invar chamber is about a few cm to 10 cm in order to give the chamber a function as a vacuum vessel. This makes it possible to obtain a magnetic shielding effect equal to that obtained when a permalloy plate about 1 mm thick is used.